Why are zooplankton attracted to point light sources?

Abstract

Zooplankton normally aggregate near the surface during the night to mate and feed. This behavior is known as Diel Vertical Migration and it is modulated by factors such as light intensity, predation risk, and turbulence. Near surface turbulence tend to result in deeper distribution of zooplankton, but it is still not clear how zooplankton assess the turbulence regime. Here, I propose an explanation where the light regime serves as a cue for zooplankton, offering information on turbulence levels near the surface, making them ascend when turbulence levels are favorably low. Steady point light sources, such as the moon, are only perceivable through a flat air-water interface, and increasing surface roughness introduces temporal and spatial noise to the light, which correlates with heightened turbulence near the surface. I test this hypothesis by exposing Ceriodaphnia sp, Daphnia magna and Temora longicornis to simulated light regimes corresponding to still, breeze and stormy conditions. Both cladocerans acted with approximately twice as high, and more directional ascent rates in response to the still light regime, although with no statistical significance for Daphnia magna. The marine copepod, Temora longicornis, however, showed a deviating pattern where the breeze light regime induced the highest migration speed, and no significant difference in mean directional change rate was observed. Field sampling of zooplankton during a full moon night showed high numbers of zooplankton during the moon culmination, but the abundance was also equally high during the morning. These findings introduce a new explanatory model for the general attraction of zooplankton to steady light sources, and suggest that the turbulence near the surface may have contributed to the evolution of their light regime dependent migratory behavior.

Popular Abstract

Why are zooplankton attracted to point light sources?

The attraction to point light sources has been documented across various Arthropoda taxa, most notably among insects. Zooplankton, however, also exhibit this behavior. The reason for that might be linked to their evolutionary strategies for optimizing feeding and mating under calmer conditions in the water surface.

Zooplankton play a pivotal role in the world's oceans. As the primary consumers of marine phytoplankton, they form the principal link to higher trophic levels. These organisms also perform spectacular daily migrations known as Diel Vertical Migration (DVM), characterized by their ascension in the water column at night for feeding and descension to deeper depths during the day. This behavior, largely regulated by light intensity, is a strategic response for mitigating the risks of predation from visually oriented predators. The DVM can also be controlled by turbulence, an important aspect of the aquatic environment that influence the structural and functional aspects of planktonic systems.

I propose that the way a point light source is perceived by zooplankton may serve as a cue, guiding their migration patterns in response to different levels of turbulence. Specifically, that the light regime which corresponds to the moonlight under calm weather will serve as a proxy for low turbulent conditions, providing an opportunity for zooplankton to aggregate at the surface, facilitating resource acquisition and mating.
Migration patterns were assessed across three zooplankton species, under three light regimes created through a LED matrix panel —still, breeze, and stormy— which are connected to turbulence intensities. I hypothesized that ‘still’ will induce a migration pattern characterized by higher vertical velocity and lower directional change rate.

Results & Discussion
Two freshwater species (Ceriodaphnia sp. and Daphnia magna) acted according to the predictions, with approximately twice as high, and more directional ascent rates in response to the still light regime, although with no statistical significance for D. magna. The marine copepod, Temora longicornis, however, showed a deviating pattern where the breeze light regime induced the highest migration speed, and no significant difference in mean directional change rate was observed. Field sampling of zooplankton during a full moon night showed high numbers of zooplankton during the moon culmination, but the abundance was also equally high during the morning.

Intuitively, we expect zooplankton to ascend to the top of the water column during the night in periods of low turbulence, when an opportunity for aggregation is offered. Addressing the central question posed in the title of this study—"why are zooplankton attracted to point light sources"— from an evolutionary point of view, optimizing feeding and mating under calmer conditions would be advantageous, as it provides higher encounter rates and optimal foraging conditions. The moon, when viewed as a point light source, can serve as a proxy for such scenario. My findings introduce a new explanatory model for the general attraction of zooplankton to steady light sources, and suggest that the turbulence near the surface may have contributed to the evolution of their light regime dependent migratory behavior. Cladocerans particularly seem to prefer calm and still conditions, making the hypothesis interesting to pursue for them. Future research should continue to explore the topic, potentially focusing more on that group, and seeking to validate findings under natural conditions, especially during periods where the presence of predators is high.


Master’s Degree Project in Biology 45 credits 2024
Department of Biology, Lund University

Advisor: Erik Selander
Functional Ecology Division

• author: Jaeger Marques, Luisa
• supervisor: Erik Selander
• organization: Degree Projects in Biology
• course: BION02 20232
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9155800
• date added to LUP: 2024-05-30 16:11:47
• date last changed: 2024-05-30 16:11:47